Series are the groupings by which archivists organize or maintain manuscript collections. They can be based on a number of organizational principles, such as document type, subject matter, or original arrangement. All three series of the Abraham Lincoln Papers are of a document type called "General Correspondence," which includes both incoming and outgoing correspondence and enclosures, such as reports, pamphlets, newspaper clippings, extracts, and copies of items. General Correspondence can also include memoranda, notes, drafts of speeches and like matter, created independently of correspondence and filed chronologically.

The three General Correspondence series of the Abraham Lincoln Papers have overlapping date ranges because they came to the collection separately. Series 1 contains the papers originally gathered by Robert Todd Lincoln, Abraham Lincoln's son. Series 2 consists of documents retained by John G. Nicolay, secretary to Lincoln during his presidency. These were returned to the collection in 1959. Series 3 is made up of items acquired by the Library of Congress later and added to the original collection. Series 1 and 3 contain items dated later than Lincoln's assassination in 1865. Examples of these are letters of condolence to Lincoln's widow, Mary Todd Lincoln, and correspondence between Robert Todd Lincoln and others. (Abraham Lincoln Papers: Provenance)